1. Field of the Invention
The present invention relates to a semiconductor substrate, a method of fabricating the same, a semiconductor device, and a method of fabricating the same. More particularly, the present invention relates to a semiconductor substrate suitable for forming a GaN layer thereon, a method of fabricating the same, a semiconductor device, and a method of fabricating the same.
2. Discussion of the Background
Light emitting diodes (LEDs) that have a gallium nitride (GaN)-based semiconductor may be used in various devices including traffic signals, backlights in a liquid crystal panel, and the like. It is known that the light emitting efficiency of LEDs is influenced by the dislocation density of crystals and defects. GaN-based semiconductor crystals may be grown on a heterogeneous substrate such as sapphire. However, lattice mismatch or a difference of thermal expansion coefficients may occur between a GaN layer and a substrate, which results in a high dislocation density or an increase in defects.
The crystal growth of a GaN-based semiconductor may be performed on a homogeneous substrate such as a GaN substrate. However, it may be difficult to form a GaN melt and to fabricate a GaN substrate due to a high dissociation rate of nitrogen in GaN, and the like. A GaN bulk crystal grown for a GaN substrate may be separated by mechanical grinding or laser ablation. However, it may be difficult to reproduce a GaN substrate of a practical size. Particularly, the laser ablation may require a considerable amount of time to perform, which increases the cost of a GaN substrate.
GaN crystal growth is shown and described in “Polycrystalline GaN for light emitter and field electron emitter applications” by S. Hasegawa, S. Nishida, T. Yamashita, and H. Asahi, (Thin Solid Layers 487 (2005) 260-267) (hereinafter “Hasegawa, et al.”), and “Buried Tungsten Metal Structure Fabricated by Epitaxial-Lateral-Overgrown GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy” M. Haino, et. al., (Jpn. J. Appl., 39 (2000) L449) (hereinafter “Haino, et. al.”), which are hereby incorporated by reference in their entirety for all purposes as if fully set forth herein. For example, GaN crystals are respectively grown on quartz substrates, high-melting-point metal substrates of Tungsten (W), Molybdenum (Mo), Tantalum (Ta), and Niobium (Nb), and Silicon (Si) substrates using plasma assisted molecular beam epitaxy.
Since it may be difficult to fabricate the GaN substrate, and GaN substrate fabrication cost may also be increased as described above, a semiconductor device such as an LED or laser diode is frequently fabricated by growing a GaN layer on a heterogeneous substrate such as sapphire. However, the light emitting performance of LEDs may be degraded due to high dislocation density or the increase of defects, as described above. Further, a sapphire substrate has a lower thermal conductivity than that of the GaN substrate and may reduce a heat dissipation property of a device. When an LED or laser diode is fabricated, using a sapphire substrate may have a negative influence on the operational lifetime of the LED or laser diode.